1. Field of the Invention
The present invention relates to a vehicular mirror device for use as an automotive door mirror device or the like and to couple a rearview mirror to a vehicle body and rotatably support it on the vehicle body.
2. Related Art
There is known, for example, a description in Japanese Utility Model Registration No. 2,548,021 as a power-operated vehicular door mirror device. The vehicular mirror device described in Japanese Utility Model Registration No. 2,548,021 (hereinafter, referred merely to as a “door mirror device”) has a mirror base on which a cylindrical support shaft is provided upright, a mirror housing rotatably supported on the support shaft, a torque transmission mechanism that conveys the torque generated at a drive the motor arranged on a mirror base to the mirror housing, and a clutch assembly provided in a torque transmission system of the torque transmission mechanism. The clutch assembly has a driving clutch disk in a ring form formed with projection strips (clutch projections), a driven clutch disk in a ring form formed with grooves (clutch grooves) for removably receiving the clutch projections, and a clutch spring for biasing the driving clutch disk toward the driven clutch disk.
In the mirror device described in Japanese Utility Model Registration No. 2,548,021, when an excessive torque is conveyed from the mirror housing to the driving clutch disk, the clutch projections of the driving clutch disk move out of the clutch grooves of the driven clutch disk. By releasing the coupling between the driving clutch disk and the driven clutch disk, the torque transmission mechanism is protected against excessive torque.
In the door mirror device as described in Japanese Utility Model Registration No. 2,548,021, cross sections along the radial direction of the clutch groove and the clutch projection are each formed in a trapezoidal form having slant surfaces at circumferential direction both ends thereof, so that, during torque transmission, the clutch projection at its one slant surface can be placed in pressure contact with one slant surface of the clutch groove. On this occasion, when excessive torque is transmitted to the driving clutch disk, the component force along the axis direction caused upon the slant surfaces of the clutch projection and clutch groove becomes greater than the bias force of the clutch spring, the clutch projection moves out of the clutch groove thereby releasing the coupling between the clutch disks.
However, by edge lines of the clutch projection between the top surface thereof and a pair of slant surfaces thereof, and edge lines between the slant surfaces and one end surface of the driving clutch disk being respectively provided so as to be directed parallel with the radial direction about the axis, as well as edge lines of the clutch groove between the bottom surface thereof and a pair of slant surfaces thereof, and edge lines between the slant surfaces and the other end surface of the driven clutch disk being respectively provided so as to be directed parallel with the radial direction, only a portion of the slant surface of the clutch projection at the outer peripheral side is locally pressure-contacted with only a portion of the slant surface of the clutch groove at the outer peripheral side during rotation of the driving clutch disk as described in Japanese Utility Model Registration No. 2,548,021.
During rotation of the driving clutch disk as above, if only a portion of the slant surface of the clutch projection at the outer peripheral side is locally pressure-contacted with only a portion of the slant surface of the clutch groove at the outer peripheral side during rotation of the driving clutch disk, there possibly encounters a phenomenon that the clutch projection and the clutch groove are firmly fixed together (metal-to-metal sticking) when operating the clutch assembly, i.e. when releasing the coupling state between the clutch disks, immediately after completing the assembling of the door mirror device. Even when an excessive torque much greater than a previously set threshold is transmitted to the driving clutch disk, there is a possibility that the coupling cannot be released between the clutch disks. This possibly applies an excessive load to the torque transmission mechanism and mirror housing and causes damages to those.
The sticking phenomenon like the above is to disappear rapidly as the number of operation cycles of the clutch assembly increases. When the clutch assembly is operated some three to four times for example, the clutch assembly is allowed to be stably operated with accuracy in values near the previously set threshold value.